In communication systems a modem is used to convert (modulate) digital signals generated by a computer into analog signals suitable for transmission over telephone lines. Another modem, located at the receiving end of the transmission, converts (demodulates) the analog signals back into digital form. The ability of modems to operate in the presence of interference is important in modern digital communication systems.
Traditional telephone voiceband modems use the same band (frequency) of transmission for upstream and downstream. However, in digital subscriber line (DSL) based systems there is a need to move to higher frequencies, and at those frequencies coupling between adjacent lines and attenuation of transmit signals at the receiver makes it preferable to use separate bands for upstream (US) and downstream (DS).
There are many potential types of interference including: (1) transmitter feed-through (Tx-FT) which is the result of the transmission signal Tx feeding back into the receive (Rx) path caused by imperfect transhybrid cancellation in 2 to 4 wire interfaces; and (2) radio frequency interference (RFI) which includes ingress from external sources using amplitude modulation (AM) for commercial radio broadcasting.
Tx-FT and RF interference can be very large and must be eliminated in order recover the desired signal with an appropriate signal-to-noise ratio (SNR). Further, due to the large attenuation in the twisted pair, Tx-FT and RF interference can be larger than the received signal, which generally requires additional filtering.
Making a communication system tolerant to interference (Tx-FT and RF) provides several benefits. In particular, an interference tolerant system can make more efficient use of spectrum by being able to share that spectrum with multiple users. Such a system also provides more reliable communications supporting higher transfer rates while utilizing the same bandwidth and power.
Analog filters are usually employed to reduce interference before digitizing the signal at the receiver. Although this provides a theoretical optimal solution, it requires very expensive analog filtering schemes with high-order filters that usually produce other associated problems such as distortion noise and the introduction of non-linearities in the signal.